1. Field of the Invention
The invention relates to frequency dividers or counters which provide programmable compensation or adjustment for oscillators.
2. Prior Art
Quartz crystal oscillators are frequently employed as time standards in watches and chronometers. Generally an integrated circuit frequency divider or counter is employed to reduce the high frequency of the crystal oscillator to lower frequencies so that seconds, minutes, hours, etc., may be displayed. Since the frequency of most crystal oscillators deviates from the desired frequency some compensation is provided, particularly where greater accuracy is required. Often a trimming capacitor is employed with the crystal oscillator to provide this compensation. The capacitance of the trimming capacitor is adjusted to provide tuning of a given crystal. The compensation provided by these capacitors is unstable; and, in watch circuits where power consumption must be low, compensation can only be provided for a limited band of crystal oscillator frequency deviations. In contrast, with the disclosed counter, trimming capacitors are not required, thus the cost of the crystal oscillator is lower, in addition the oscillator power consumption may be reduced. Moreover, more accurate and more stable compensation is provided, and crystal oscillators with larger frequency deviations may be used, without loss of accuracy.
Compensating counters or dividers have been employed to correct crystal oscillator frequency deviations. In general, these devices either provide faster counting or suppress counts for compensation. One such circuit is described in U.S. Pat. No. 3,945,194. Also, in the prior art section of this patent, numerous other circuits are described.
With the invented programmable counter both "plus" and "minus" programming is possible, thus both fast or slow crystal oscillators may be employed. Additionally the described circuit may be readily expanded to correct the oscillator output to any desired degree of accuracy.